Uniform bottom spacer for vfet devices

ABSTRACT

Vertical field effect transistor (VFET) structures and methods of fabrication include a bottom spacer having a uniform thickness. The bottom spacer includes a bilayer portion including a first layer formed of an oxide, for example, and a second layer formed of a nitride, for example, on the first layer, and a monolayer portion of a fourth layer of a nitride for example, immediately adjacent to and intermediate the fin and the bilayer portion.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.16/039,472, entitled “UNIFORM BOTTOM SPACER FOR VFET DEVICES,” filedJul. 19, 2018 incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates in general to semiconductor devicefabrication methods and resulting structures. More specifically, thepresent invention relates to fabrication methods and resultingsemiconductor device structures including a novel design for a uniformbottom spacer of a vertical transport field effect transistor.

In contemporary semiconductor device fabrication processes, a largenumber of semiconductor devices, such as field effect transistors(FETs), are fabricated on a single wafer. Some non-planar transistordevice architectures, such as vertical field effect transistors (VFETs),employ semiconductor fins channels and side-gates that can be contactedoutside the active region, resulting in increased device density andperformance over lateral devices. VFETs are one of the promisingalternatives to standard lateral FET structures due to benefits, amongothers, in terms of reduced circuit footprint. In this type ofstructure, the current flow is perpendicular to a supporting wafer,unlike the lateral current flow in fin-type FETs (FinFETs). When formingVFETs, spacers need to be provided between and around the vertical finchannel to isolate the bottom source or drain (S/D) region and the topS/D region.

SUMMARY

According to one or more embodiments of the present invention, verticalfield effect transistor (VFET) structures and methods for forming abottom spacer in a VFET structure are provided. A non-limiting exampleof the method of forming a bottom spacer layer in the vertical fieldeffect transistor (VFET) structure in accordance with one or moreaspects of the present invention includes forming one or more verticallyoriented fins on a substrate. A first layer is conformally depositedonto the substrate. A second layer is conformally deposited onto thefirst layer, and a third layer is conformally deposited on the secondlayer. The third layer is selectively removed so as to completely removethe third layer from sidewalls and top surfaces of the one or morevertically oriented fins. The second layer is selectively removed so asto remove the second layer from sidewalls and top surfaces of the one ormore vertically oriented fins, wherein selectively removing the secondlayer forms a recess immediately adjacent the one or more verticallyoriented fins. The first layer and remaining portions of the third layerare selectively removed to the second layer, and a fourth layer isconformally deposited onto the substrate filling the recess immediatelyadjacent the one or more vertically oriented fins. The fourth layer isselectively removed from the sidewalls and top surfaces of the one ormore vertically oriented fins to form a monolayer portion of the fourthlayer intermediate the one or more vertically oriented fins and abilayer portion of the layer and second layer.

A non-limiting example of the method of forming a bottom spacer layer inthe vertical field effect transistor (VFET) structure in accordance withone or more aspects of the present invention includes forming one ormore vertically oriented fins on a substrate, wherein each of the one ormore vertically oriented fins includes a hardmask of siliconborocarbonitride on a top surface thereof. A first oxide layer and afirst nitride layer are conformally deposited on the first oxide layerhaving a substantially uniform thicknesses by atomic layer deposition onthe substrate. A second oxide layer having a variable thickness isdeposited on the first nitride layer by high density plasma. Exposedportions of the second oxide layer are selectively removed to the firstnitride layer so as to completely remove the second oxide layer fromsidewalls and top surfaces of the one or more vertically oriented fins.Exposed portions of the first nitride layer are selectively removed tothe first and second oxide layers so as to remove the first nitridelayer from the sidewalls and top surfaces of the one or more verticallyoriented fins, wherein selectively removing the first nitride layerforms a recess immediately adjacent the one or more vertically orientedfins. Exposed portions of the first oxide layer and remaining portionsof the second oxide layer are selectively removed to the first nitridelayer, and a second nitride layer is conformally deposited onto thesubstrate filling the recess immediately adjacent the one or morevertically oriented fins. The second nitride layer is selectivelyremoved from sidewalls and top surfaces of the one or more verticallyoriented fins to form a monolayer portion of the second nitride layerimmediately adjacent to and intermediate the one or more verticallyoriented fins and a bilayer portion of the first oxide layer and firstnitride layer.

A non-limiting example of a VFET structure in accordance with one ormore aspects of the present invention includes one or more verticallyoriented fins extending from a substrate. A bottom spacer layer is onthe substrate and has a uniform thickness that is less than a verticalheight of the one or more vertically oriented fins. The bottom spacerlayer includes a bilayer portion and a monolayer portion of a nitride.The bilayer portion includes an oxide layer and a nitride layer on theoxide layer. The monolayer portion of the nitride is intermediate thefin and the bilayer portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present invention is particularly pointed outand distinctly defined in the claims at the conclusion of thespecification. The foregoing and other features and advantages areapparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

PRIOR ART FIG. 1 is a scanning electron micrograph of a prior artvertical field effect transistor (VFET) structure up to formation of abottom spacer layer exhibiting local thickness variations between fins;

PRIOR ART FIG. 2 is a scanning electron micrograph of a prior art VFETstructure up to formation of a bottom spacer layer exhibiting thicknessvariations as a function of microloading effects;

FIG. 3 depicts the cross-sectional view of the VFET structure up toformation of one or more vertically oriented fins according to one ormore embodiments of the present invention;

FIG. 4 depicts the cross-sectional view of the VFET structure of FIG. 3subsequent to conformal deposition of a first layer by atomic layerdeposition according to one or more embodiments of the presentinvention;

FIG. 5 depicts the cross-sectional view of the VFET structure of FIG. 4subsequent to conformal deposition of a second layer by atomic layerdeposition according to one or more embodiments of the presentinvention;

FIG. 6 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 5 subsequent to non-conformal deposition of a thirdlayer by high density plasma according to one or more embodiments of thepresent invention;

FIG. 7 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 6 subsequent selective removal of the second layerfrom sidewalls and top surfaces of the fins according to one or moreembodiments of the present invention;

FIG. 8 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 7 subsequent to selective removal of the second layerfrom a bottom surface of the substrate to form a recess immediatelyadjacent each of fins according to one or more embodiments of thepresent invention;

FIG. 9 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 8 subsequent selective removal of the third layeraccording to one or more embodiments of the present invention;

FIG. 10 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 9 subsequent to conformal deposition of a fourth layerby atomic layer deposition filling the recess according to one or moreembodiments of the present invention; and

FIG. 11 depicts an enlarged cross-sectional view of the of the VFETstructure of FIG. 10 subsequent to selective removal of the fourth layerfrom sidewalls and top surface of the fins so as to form the bottomspacer according to one or more embodiments of the present invention

DETAILED DESCRIPTION

The present invention is generally directed to VFET structures andmethods to form the VFET structures. More particularly, embodiments ofthe present invention form a substantially uniform bottom spacerincluding a nitride/oxide bilayer portion and a silicon borocarbonitride(SiBCN) hardmask. In embodiments of the invention, the substantiallyuniform bottom spacer can further include a nitride monolayer portionimmediately adjacent the fins.

Turning now to a more detailed description of technologies relevant tothe present invention, known processes for forming the bottom spacer inVFET structures utilize a combination of atomic layer deposition (ALD)and high density plasma (HDP) to deposit a silicon dioxide layer so asto form the bottom spacer. ALD provides a highly conformal layer whereasHDP is non-conformal, i.e., less than 100% conformal. An etchbackprocess using an etchant selective to the silicon dioxide is then usedto remove the silicon dioxide from the sidewalls of the fins, whereinthe top fin at this stage of fabrication includes a hardmask so as toprotect the critical dimensions of the fin during processing.Oftentimes, the etchant for selective silicon dioxide removal is abuffered hydrofluoric acid solution (BHF) and the hardmask is typicallysilicon nitride. One of the problems with current processes of record isthat the HDP deposition of silicon dioxide is space sensitive resultingin local thickness variations. FIG. 1 provides a micrograph illustratingthe thickness variation between fins subsequent to ALD depositionfollowed by HDP deposition of silicon dioxide onto a substrate includingan array of vertically oriented fins. FIG. 2 provides a micrographshowing the microloading effect resulting from the above noted ALD/HDPdeposition of silicon dioxide subsequent to an etchback process. Asshown, silicon dioxide thickness is markedly less in isolated regionsand variable in denser fin regions. The resulting thickness variationscan lead to threshold voltage (Vt) shifts due to high k gate dielectricinteractions with the oxide bottom spacer. The resulting thicknessnon-uniformity of the bottom spacer is clearly evident in theillustrated examples. In some prior processes of record, HDP can be usedto deposit a nitride layer for the bottom spacer. However, sidewallstrip from the fins subsequent to HDP deposition of the nitride layerremains problematic because of hardmask erosion, which are alsotypically formed of a nitride making etch selectivity difficult. As aresult, the process window to minimize erosion of the hardmask isrelatively narrow.

As will be described in greater detail herein, in embodiments of thepresent invention ALD is used to form a thin conformal layer of silicondioxide on the fins followed by ALD deposition of a conformal layer ofsilicon nitride to form a bilayer. HDP is then used to deposit oxideonto the bilayer with an etchback process that results in localthickness variations between fins and microloading in isolated regionssimilar to what has been observed in prior art processes However, thesilicon nitride is selectively removed from the fin sidewalls by a wetetch process that is followed by a dry etch process selective to theoxide to the bottom surface to form a recess immediately adjacent thefins. The remaining HDP deposited oxide is then selectively removedresulting in a bilayer of ALD nitride and ALD oxide and the recessimmediately adjacent the fins. A conformal layer of ALD silicon nitrideis then deposited filling the space followed by etchback of the siliconnitride to fill the recessed space with a monolayer of silicon nitride.The nitride filled recess and the bilayer have the same thickness,thereby providing a substantially uniform bottom spacer. The uniformbottom spacer overcomes the problems noted in the prior art including,without limitation, eliminating the microloading effects, eliminatinglocal thickness variations between fins, and because the HDP oxide doesnot directly touch the hardmask on the fins during the process offorming the bottom spacer, no hardmask erosion is observed. Moreover,because a nitride bottom monolayer is used in the recess immediatelyadjacent the fins, threshold voltage issues are minimized.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. It is notedthat various connections and positional relationships (e.g., over,below, adjacent, etc.) are set forth between elements in the followingdescription and in the drawings. These connections and/or positionalrelationships, unless specified otherwise, can be direct or indirect,and the present invention is not intended to be limiting in thisrespect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising”, “includes”, “including”, “has,”“having”, “contains” or “containing”, or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic.Moreover, such phrases are not necessarily referring to the sameembodiment. Further, when a particular feature, structure, orcharacteristic is described in connection with an embodiment, it issubmitted that it is within the knowledge of one skilled in the art toaffect such feature, structure, or characteristic in connection withother embodiments whether or not explicitly described.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements. It should benoted, the term “selective to”, such as, for example, “a first elementselective to a second element”, means that a first element can be etchedand the second element can act as an etch stop.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication are not bedescribed in detail herein. Moreover, the various tasks and processsteps described herein can be incorporated into a more comprehensiveprocedure or process having additional steps or functionality notdescribed in detail herein. In particular, various steps in themanufacture of semiconductor devices and semiconductor-based ICs arewell known and so, in the interest of brevity, many conventional stepswill only be mentioned briefly herein or will be omitted entirelywithout providing the well-known process details.

By way of background, however, a more general description of thesemiconductor device fabrication processes that can be utilized inimplementing one or more embodiments of the present invention will nowbe provided. Although specific fabrication operations used inimplementing one or more embodiments of the present invention can beindividually known, the described combination of operations and/orresulting structures of the present invention are unique. Thus, theunique combination of the operations described in connection with thefabrication of a semiconductor device utilizing a non-selective lowtemperature deposition process for forming the top source/drain in aVFET device followed by a low temperature oxidation process to form thenon-uniform top spacer according to the present invention utilizes avariety of individually known physical and chemical processes performedon a semiconductor (e.g., silicon) substrate, some of which aredescribed in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, and atomic layer deposition (ALD) among others.

Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Reactive ionetching (RIE), for example, is a type of dry etching that useschemically reactive plasma to remove a material, such as a maskedpattern of semiconductor material, by exposing the material to abombardment of ions that dislodge portions of the material from theexposed surface. The plasma is generated under low pressure (vacuum) byan electromagnetic field.

Semiconductor doping is the modification of electrical properties bydoping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (RTA).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device.

Semiconductor lithography is the formation of three-dimensional reliefimages or patterns on the semiconductor substrate for subsequenttransfer of the pattern to the substrate. In semiconductor lithography,the patterns are formed by a light sensitive polymer called aphoto-resist. To build the complex structures that make up a transistorand the many wires that connect the millions of transistors of acircuit, lithography and etch pattern transfer steps are repeatedmultiple times. Each pattern being printed on the wafer is aligned tothe previously formed patterns and slowly the conductors, insulators andselectively doped regions are built up to form the final device.

As used herein, “p-type” refers to the addition of impurities to anintrinsic semiconductor that creates deficiencies of valence electrons.In a silicon-containing substrate, examples of p-type dopants, i.e.,impurities, include but are not limited to: boron, aluminum, gallium andindium.

As used herein, “n-type” refers to the addition of impurities thatcontributes free electrons to an intrinsic semiconductor. In a siliconcontaining substrate examples of n-type dopants, i.e., impurities,include but are not limited to antimony, arsenic and phosphorous.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about,”“substantially,” “approximately,” and variations thereof, can include arange of ±8% or 5%, or 2% of a given value.

FIGS. 3-10 schematically illustrates cross-sectional viewsrepresentative of a process for forming a uniform bottom spacer for aVFET structure 100 according to aspects of the invention. It should beapparent that the present invention is not limited to this particularstructure nor is it intended to be limited to any particular method forforming the VFET up to formation of the fins.

FIG. 3 is a cross sectional view illustrating the VFET structure 100 upto formation of the fins 102, there of which are shown. The fins 102 canbe formed from an epitaxy region 104 formed on or part of a substrate,wherein the bottom epitaxy region 104 can be doped (n-type or p-type) todefine a bottom source/drain region (not shown). The doping of thebottom epitaxy region 104 can be configured as the bottom source/drainby a variety of methods, such as, for example, diffusion and/or ionimplantation, in-situ doped epitaxy, or any other suitable dopingtechniques.

The substrate can be any suitable substrate material, such as, forexample, any semiconductor material including, but not limited to, Si,Ge, SiGe, SiC, SiGeC, II/IV, and III/V compound semiconductors such as,for example, InAs, GaAs, and InP. Multilayers of these semiconductormaterials can also be used as substrate. In one or more embodiments andwhen substrate is a remaining semiconductor material portion of a bulksemiconductor substrate, the substrate can be of a single crystallinesemiconductor material, such as, for example, single crystallinesilicon. In some embodiments, the crystal orientation of the remainingsemiconductor portion of the bulk semiconductor substrate can be {100},{110}, {111} or any other of the well-known crystallographicorientations. As will be described in greater detail below, eachsemiconductor fin can include the same semiconductor material, or adifferent semiconductor material, from substrate.

In another embodiment, substrate includes at least an insulator layer ofa semiconductor-on-insulator (SOI) substrate (not specifically shown).Although not specifically shown, one skilled in the art understands thatan SOI substrate includes a handle substrate, an insulator layer locatedon an upper surface of the handle substrate, and a semiconductor layerlocated on an uppermost surface of the insulator layer. The handlesubstrate provides mechanical support for the insulator layer and thesemiconductor layer. The semiconductor layer of such an SOI substratecan be processed into semiconductor fins.

The handle substrate and the semiconductor layer of the SOI substratecan include the same, or different, semiconductor material. The term“semiconductor” as used herein in connection with the semiconductormaterial of the handle substrate and the semiconductor layer denotes anysemiconductor material including, for example, Si, Ge, SiGe, SiC, SiGeC,II/VI, and III/V compound semiconductors such as, for example, InAs,GaAs, or InP. Multilayers of these semiconductor materials can also beused as the semiconductor material of the handle substrate and thesemiconductor layer. In one or more embodiments, the handle substrateand the semiconductor layer are both formed of silicon. In someembodiments, the handle substrate is a non-semiconductor materialincluding, for example, a dielectric material and/or a conductivematerial. In yet other embodiments, the handle substrate can be omittedand the substrate 104 includes only an insulator layer.

In one or more embodiments, the handle substrate and the semiconductorlayer can have the same or different crystal orientation. For example,the crystal orientation of the handle substrate and/or the semiconductorlayer can be {100}, {110}, or {111}. Other crystallographic orientationsbesides those specifically mentioned can also be used in the presentapplication. The handle substrate and/or the semiconductor layer of theSOI substrate can be a single crystalline semiconductor material, apolycrystalline material, or an amorphous material. Typically, at leastthe semiconductor layer is a single crystalline semiconductor material.

The insulator layer of the SOI substrate and that can be employed assubstrate can be a crystalline or non-crystalline oxide and/or nitride.In one embodiment, the insulator layer is an oxide such as, for example,silicon dioxide. In another embodiment, the insulator layer is a nitridesuch as, for example, silicon nitride or boron nitride. In yet anotherembodiment, the insulator layer is a multilayered stack of, in anyorder, silicon dioxide and one of silicon nitride or boron nitride.

The SOI substrate can be formed utilizing standard processes includingfor example, SIMOX (Separation by IMplantation of OXygen) or layertransfer. When a layer transfer process is employed, an optionalthinning step can follow the bonding of two semiconductor waferstogether. The optional thinning step reduces the thickness of thesemiconductor layer to a layer having a thickness that is moredesirable.

By way of example, the thickness of the semiconductor layer of the SOIsubstrate can be from 10 nm to 100 nm. Other thicknesses that are lesserthan, or greater than, the aforementioned thickness range can also beused as the thickness of the semiconductor layer of the SOI substrate.The insulator layer of the SOI substrate can have a thickness from 1 nmto 200 nm. Other thicknesses that are lesser than, or greater than, theaforementioned thickness range can also be employed as the insulatorlayer.

The first exemplary semiconductor structure shown in FIG. 3 can beformed by first providing a bulk semiconductor substrate (as definedabove) or a SOI substrate (as defined above). Adjacent devices formed onthe substrate can be separated by shallow trench isolation regions (notshown). The shallow trench isolation regions can be created early duringthe semiconductor device fabrication process, e.g., before thetransistors such as the illustrated VFET are formed. The key steps forforming the shallow trench isolation regions typically involve etching apattern of trenches in the substrate, depositing one or more dielectricmaterials (such as silicon dioxide) to fill the trenches, and removingthe excess dielectric using a technique such as chemical-mechanicalplanarization.

The bottom epitaxy layer 104 can be formed by epitaxial growth and/ordeposition. As used herein, the terms “epitaxial growth and/ordeposition” and “epitaxially formed and/or grown” mean the growth of asemiconductor material (crystalline material) on a deposition surface ofanother semiconductor material (crystalline material), in which thesemiconductor material being grown (crystalline overlayer) hassubstantially the same crystalline characteristics as the semiconductormaterial of the deposition surface (seed material). In an epitaxialdeposition process, the chemical reactants provided by the source gasesare controlled and the system parameters are set so that the depositingatoms arrive at the deposition surface of the semiconductor substratewith sufficient energy to move about on the surface such that thedepositing atoms orient themselves to the crystal arrangement of theatoms of the deposition surface. Therefore, an epitaxially grownsemiconductor material has substantially the same crystallinecharacteristics as the deposition surface on which the epitaxially grownmaterial is formed. For example, an epitaxially grown semiconductormaterial deposited on a {100} orientated crystalline surface will takeon a {100} orientation. In some embodiments, epitaxial growth and/ordeposition processes are selective to forming on semiconductor surface,and generally do not deposit material on exposed surfaces, such assilicon dioxide or silicon nitride surfaces.

In one or more embodiments, the gas source for the deposition ofepitaxial semiconductor material can include a silicon containing gassource, a germanium containing gas source, or a combination thereof. Forexample, an epitaxial Si layer can be deposited from a silicon gassource that is selected from the group consisting of silane, disilane,trisilane, tetrasilane, hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, methylsilane, dimethylsilane,ethylsilane, methyldisilane, dimethyldisilane, hexamethyldisilane andcombinations thereof. An epitaxial germanium layer can be deposited froma germanium gas source that is selected from the group consisting ofgermane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. An epitaxial silicongermanium alloy layer can be formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium and argon can beused. The particular epitaxial region is not intended to be limited andwill generally depend on the type of VFET being formed.

The epitaxial deposition can be carried out in a chemical vapordeposition apparatus, such as a metal organic chemical vapor deposition(MOCVD) apparatus or a plasma enhanced chemical vapor deposition (PECVD)apparatus. The temperature for epitaxial deposition typically rangesfrom 500° C. to 900° C.

The vertically oriented semiconductor fins 102 are formed on and coupledto the bottom epitaxy layer 102. Any known composition and manner offorming the semiconductor fins 102 can be utilized. In one or moreembodiments, an undoped channel region is epitaxially deposited over thebottom epitaxy layer 102 and is etched using a patterned hard mask 106to form a plurality of fins, as described below. The fins 106, three ofwhich are shown, extend vertically from the bottom epitaxy layer 102.Stated differently, the fins 102 are normal to or perpendicular to thesubstrate including a portion having a shape of a rectangularparallelepiped.

The etching to form the fins 102 can include a dry etching process suchas, for example, reactive ion etching, plasma etching, ion etching, orlaser ablation. The etching can further include a wet chemical etchingprocess in which one or more chemical etchants are used to removeportions of the blanket layers that are not protected by the patternedhardmask 106.

The direction along which a semiconductor fin 102 laterally extends themost is herein referred to as a “lengthwise direction” of the fin. Theheight of each semiconductor fin 102 can be in a range from about 5 nmto about 300 nm, although lesser and greater heights can also beemployed. The width of each semiconductor fin 102 can be in a range fromabout 5 nm to about 100 nm, although lesser and greater widths can alsobe employed. In various embodiments, the fins 102 can have a width inthe range of about 4 nm to about 20 nm, or can have a width in the rangeof about 5 nm to about 15 nm, or in the range of about 6 nm to about 8nm. In various embodiments, the fin 102 can have a height in the rangeof about 25 nm to about 75 nm, or in the range of about 40 nm to about50 nm. Alternatively, the bottom source/drain regions can be formedafter the formation of fins.

Multiple fins 102 can be arranged such that the multiple fins 102 havethe same lengthwise direction, and are laterally spaced from each otheralong a horizontal direction that is perpendicular to the lengthwisedirection. In this case, the horizontal direction that is perpendicularto the common lengthwise direction is referred to as a “widthwisedirection.” Each fin 102 includes a pair of parallel sidewalls along thelengthwise direction.

The fin width and the fin pitch can vary in different areas of a finarray, and can vary from one fin array to another on a semiconductorwafer, according to the design parameters of the integrated circuit thatis being made. For example, fins of negatively doped FinFETs can have adifferent fin size than positively doped FinFETs because of theelectrical properties of the materials.

The hardmask 106 can include, for example, a silicon borocarbonitride(SiBCN) hardmask. The hardmask 106 can be deposited using a depositionprocess, including, but not limited to, PVD, CVD, PECVD, or anycombination thereof.

FIG. 4 illustrates the VFET structure 100 of FIG. 3 subsequent to ALD ofa conformal first layer 108 of an oxide, for example. In one or moreembodiments, the conformal first layer 108 is at a thickness of lessthan about 10 nanometers (nm); in one or more other embodiments, theconformal first layer of 108 is at a thickness of less than about 5 nm;and in still one or more other embodiments, the conformal first layer108 is at a thickness of about 3 nm or as thin as 1 nm. As noted above,deposition of the first layer 108 by ALD produces a highly conformallayer. ALD is a thin film growth technique that uses alternating andsaturation reactions to deposit films in a layer by layer fashion. Byrepeating this reaction sequence, highly conformal films of anythickness, e.g., monolayers to microns, can be deposited with atomiclayer precision.

FIG. 5 illustrates the VFET structure 100 of FIG. 4 subsequent to ALDdeposit of a conformal second layer 110 that is of a different materialfrom the first layer, e.g., a nitride. The deposited second layer, e.g.,silicon nitride, as well as the first layer are of uniform thicknessbecause of atomic layer deposition. In one or more embodiments, theconformal second layer 110 of nitride, for example, is at a thickness ofless than about 20 nanometers (nm); in one or more other embodiments,the conformal second layer 110 is at a thickness of less than about 10nm; and in still one or more other embodiments, the conformal secondlayer 110 is at a thickness of about 3 nm to 10 nm. In one or moreembodiments, the thickness of the conformal second layer 110 is at leasttwice the thickness of the conformal first layer 108.

FIG. 6 illustrates the VFET structure 100 of FIG. 5 subsequent to HDPdeposition of a non-conformal third layer 112, can be an oxide the sameas or different oxide compared to the first layer, e.g., silicondioxide, followed by etchback selective to the second layer 110 so as toremove only the third layer from the sidewalls and top surface of thefins. As shown, thickness variations of the remaining third layer 112subsequent to the etchback process is evident as microloading effectsare observed in less dense regions and local thickness variations areshown between fins.

FIG. 7 illustrates the VFET structure 100 of FIG. 6 subsequent toselective etching selective to the third layer 112 of the exposed secondlayer 110 from the sidewalls and top surfaces of the fins and theexposed portions of second layer 110 immediately adjacent the fins tocreate a recess 114. It should be noted that the term “selective to,”such as, for example, “a first element selective to a second element,”means that the first element can be etched and the second element canact as an etch stop. By way of example, a mixture of phosphoric acid(H₃PO₄) and water at elevated temperatures of about 140 to about 180° C.can be used to selectively etch a second layer of nitride 110 ratherthan third layer 112 formed of an oxide.

FIG. 8 illustrates the VFET structure 100 of FIG. 7 subsequent toetching selective to the third layer 112 of the exposed portion of thesecond layer 110 immediately adjacent the fins to create a recess 116,which exposes the bottom epitaxial surface 104. By way of example, a dryetch process selective to the third layer can be used to create therecess 116 immediately adjacent to the fins.

FIG. 9 illustrates the VFET structure 100 of FIG. 8 subsequent toetching selective to the second layer or silicon (e.g., the fin channelregion) to remove exposed portions of first and third layers 108 and112. In one or more embodiments, a wet etchant solution is applied. Byway of example, the etchant solutions for selective removal of oxidescan include HNO₃, HCL, H₂SO₄, HF or combinations thereof. By way ofexample, the wet etching process can be a buffered oxide etch, which isalso known as a buffered HF or BHF. BHF is a mixture of a bufferingagent, such as ammonium fluoride (NH₄F), and hydrofluoric acid (HF). HClcan be added to the BHF solutions if needed to dissolve any insolubleproducts that can form during the etch back.

A common buffered oxide etch solution includes a 6:1 volume ratio of 40%NH₄F in water to 49% HF in water. The buffered oxide etch solution willetch the oxide at approximately 2 to 4 nanometers per second at 25degrees Celsius. Temperature can be increased as needed to raise theetching rate.

The resulting VFET structure 100 includes a bilayer of the remaining ALDdeposited second layer 110 and the underlying ALD deposited first layer108, which is protected by the second layer 110, and recesses 118immediately adjacent the fins 102. Because the first layer 108 andsecond layers 110 were deposited by ALD, the thicknesses are uniformbetween the fin and in less dense regions.

FIG. 10 illustrates the VFET structure 100 of FIG. 9 subsequent to ALDdeposition of a conformal layer of a fourth layer 120, which can be ofthe same class of material as the second layer, e.g., nitrides. The ALDdeposition is continued to provide a thickness effective to fill therecess 118 shown in FIG. 10 with the fourth layer. The thickness of thefourth layer deposited is generally less than 10 nm. There is no damageto the hardmask 106 and the ALD provides a very conformal coatingthereon. The conformal fourth layer 120 can be the same as or adifferent nitride than second layer 110, for example.

FIG. 11 illustrates the VFET structure 100 of FIG. 10 subsequent to anetch back process. The etchback process can include applying a wetetchant solution selective to the SiBCN hardmask and removes the fourthlayer from the exposed surfaces. For example, a fourth layer formed ofsilicon nitride can be selectively removed using be selectively removedusing a solution of hydrofluoric acid diluted with ethylene glycol(HFEG). The resulting bottom spacer for the VFET structure includes abilayer portion of the first and second layers and a monolayer portionof the fourth layer immediately adjacent to and intermediate the finsand the bilayer portion. Advantageously, the bottom spacer and method offorming the bottom spacer in the VFET structure 100 eliminatesmicroloading and local variation issues, provides a monolayer portion ofa nitride, for example, immediately adjacent the fins, which eliminatesany threshold voltage issues, and because the HDP deposited third layerdoes not directly touch the hardmask, there is no hardmask damage.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described herein. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments described herein.

What is claimed is:
 1. A vertical field effect transistor (VFET)structure comprising: one or more vertically oriented fins extendingfrom a substrate; and a bottom spacer layer on the substrate having auniform thickness less than a vertical height of the one or morevertically oriented fins, the bottom spacer layer comprising a bilayerportion including an oxide layer and a nitride layer on the oxide layer,and a monolayer portion of a nitride intermediate the fin and thebilayer portion.
 2. The VFET structure of claim 1, wherein the nitridelayer in the bilayer portion is at least twice a thickness of the oxidelayer.
 3. The VFET structure of claim 1, wherein the nitride in themonolayer portion is different from the nitride in the bilayer portion.4. The VFET structure of claim 1, wherein the bilayer portion and themonolayer portion are deposited by atomic layer deposition.
 5. The VFETstructure of claim 1, wherein each of the one or more verticallyoriented fins comprises a silicon borocarbonitride hardmask.
 6. The VFETstructure of claim 1, wherein the substrate comprises an epitaxy region,wherein the one or more vertically oriented fined extend from theepitaxy region.
 7. The VFET structure of claim 6, wherein the epitaxyregion comprises dopants to define a bottom source/drain region for theVFET.
 8. The VFET structure of claim 1, wherein the one or morevertically oriented fins extending from the substrate have a heightdimension of about 5 nanometers (nm) to about 300 nm, and a widthdimension of about 5 nm to about 100 nm.
 9. The VFET structure of claim1, wherein the oxide layer is at a thickness of less than about 10 nm.10. The VFET structure of claim 1, wherein the oxide layer comprisessilicon dioxide.
 11. The VFET structure of claim 1, wherein the nitridelayer comprises silicon nitride.
 12. The VFET structure of claim 1,wherein the nitride layer is at a thickness of less than 20 nm.
 13. TheVFET structure of claim 1, wherein the monolayer portion comprisessilicon nitride.
 14. The VFET structure of claim 1, wherein the nitridelayer in the bilayer and the nitride in the monolayer are differentnitrides.
 15. The VFET structure of claim 1, wherein the nitride layerin the bilayer and the nitride in the monolayer are the same nitride.16. The VFET structure of claim 1, wherein the one or more verticallyoriented fins extending from the substrate have a variable pitch.